Since the discovery of useful applications for hydrocarbon substances there has been continuing exploration and development efforts to locate subterranean hydrocarbon deposits. These exploration and development efforts have resulted in the exhaustion of many hydrocarbon deposits and has caused an ever escalating effort to locate and exploit deposits which heretofore were difficult to discover or uneconomical to exploit. One example of such deposits are thin bed hydrocarbon deposits. Thin bed hydrocarbon deposits, some of which may be as thin as six (6) inches wide, can be the source of economical quantities of hydrocarbons especially when multiple thin bed deposits occur in the same formation or in a formation which also includes other hydrocarbon deposits. Unfortunately thin bed deposits have been and continue to be difficult to locate even when a well is drilled through the thin bed deposit. By failing to recognize such thin bed deposits producers have abandoned wells which may have proved productive, and have failed to explore in areas that may contain thinly-bedded hydrocarbon deposits because of the difficulty in locating such thin bed deposits and the economic risk of drilling dry holes.
In an effort to locate the presence of hydrocarbons in subterranean formations, a number of methods have been used. One method used in an attempt to locate thin bed hydrocarbon deposits is to insert a high vertical resolution logging tool into the well after it is drilled, and which is capable of measuring the electrical properties of geological formations with the resolution of a few inches. Unfortunately, it is uneconomical to utilize high vertical resolution logging tools unless the producer is aware of the presence of thinly-bedded formations. Another method currently used is to drill wells utilizing Measurement While Drilling (MWD) tools which are capable of evaluating characteristics of geological formations as the drilling is conducted. One class of MWD tools which is commonly used to locate hydrocarbon reserves, but has not been utilized heretofore to locate thinly-bedded reserves, is the Electro-Magnetic (EM) logging tools which also measure certain electrical characteristics of geologic formations, but with a much wider resolution. U.S. Pat. No. 4,940,943 entitled "Method and Apparatus for Optimizing the Reception Pattern of the Antenna of a Propagating Electromagnetic Wave Logging Tool" issued Jul. 10, 1990 to R. Bartel and P. Rodney discloses one such EM logging tool.
Another such EM tool is an MWD induction logging sensor. This device consists of at least one transmitting and two receiving antennas mounted on a drill collar. Electromagnetic waves propagate outward front the transmitting antenna and the phase difference and amplitude ratio of the induced voltages at the receiving antennas are measured. This data allows the calculation of the geological formation's resistivity. Because hydrocarbons have a relatively high resistivity when compared to other substances, including water and shale, areas of high resistivity in geological formations may indicate the presence of hydrocarbon deposits.
One of the physical properties sometimes seen by EM logging tools is a property called electrical anisotropy (an anisotropic medium has different physical properties when measured in different directions). Electrical anisotropy is the property where a material produces different resistivities when measured in different directions relative to the geological formation, generally parallel and perpendicular to laminated geological formations. It has been discovered that one source of electrical anisotropy shown by EM logging tools originates from the measurement across multiple alternating thin subterranean beds or laminations of different resistivity. This results from the fact that current flowing perpendicular to the bedding must pass through the more resistive layers in the sequence and therefore experiences greater resistance compared to current flowing parallel to the bedding which concentrates in the more conductive beds. This anisotropic behavior is termed macroscopic anisotropy, because it results from a measurement across a sequence of laminated beds. Microscopic anisotropy, on the other hand, is where individual beds exhibit anisotropic behavior.
Anisotropy is infrequently seen in vertical drilling because standard EM logging tools have been designed to measure the parallel resistivity of formations when the formations lie perpendicular to the well (horizontal). In many instances diverted wells are used to extend the area of a formation which can be produced from a single well pad, platform or the like. Such wells may be and frequently are drilled at a slight angle (0.degree.-25.degree.) to the formation to result in positioning the well to end at a desired location or to provide a length of the wellbore in a formation of interest. Therefore, when individual layers are neither delineated nor resolved by a logging tool and the well bore is at a deviated angle to a formation, the tool responds to the formation as if it were a macroscopically anisotropic formation; in other words, when the bedding is much thinner than the tool resolution, the tool sees the structure as a homogeneous but macroscopically anisotropic formation. The 2 MHz induction logging tool has been shown to have particular abilities to measure the anisotropy of geologic formations in such wells. An article "Response of 2 MHz Resistivity Devices in Thinly Laminated Formations (Anisotropic Resistivity and EM Log Interpretation)" by T. Hagiwara, SPE 28426, presented at the 69th Annual Technical Conference on Sep. 25-28, 1994, provides a detailed description of this response. Other EM logging tools such as electric resistivity tools and toroidal resistivity tools are also able to detect anisotropic resistivity.
As a result of recent advances in drilling technology, hydrocarbon exploration wells are being drilled utilizing directional drilling techniques and MWD sensors to evaluate geological formations. Many current wells are being drilled horizontally or at a high deviation angle from traditional vertical wells. This non-vertical drilling may allow a number of wells to be drilled from a single on-shore or offshore location into a long pay (hydrocarbon rich) zone which can result in greater hydrocarbon extraction. One of the side effects of non-vertical drilling has been a greater observation of anisotropic effects in logging data particularly in wells that are drilled between 0.degree. and 25.degree. from the plane of the formation.
Producers generally utilize a resistivity logging analysis to detect the presence of hydrocarbons which was developed in connection with drilling traditional vertical wells. Because of the construction of various EM logging tools, the measured resistivity of geological formations in vertical wells principally measures parallel resistivity. Since anisotropy results in different measurements for parallel and perpendicular resistivity of anisotropic geological formations, measurements in deviated wells have shown measured resistances which were higher than expected from vertical well data. As a result, manufacturers of EM logging tools have attempted, and continue to attempt, to correct logging data to remove the presence of anisotropy in logging data.
While an article "Response of 2 MHz Resistivity Devices in Thinly Laminated Formations (Anisotropic Resistivity and EM Log Interpretation)" by T. Hagiwara, SPE 28426, presented at the 69th Annual Technical Conference, SPE, Sep. 25-28, 1994 discloses the utilization of anisotropy to determine sand lamina resistivity and net/gross ratio (the sand thickness to total formation thickness) in a sand/shale sequence, anisotropic data is still considered something that should be corrected for instead of utilized.
Since the recovery of hydrocarbons from thin bed reserves may prove to be the source of economical quantities of hydrocarbons and may result in fewer dry holes, a continuing search has been directed to the development of economical means to identify thin bed hydrocarbon reserves in subterranean formations.